Routine Immunisation and Under-Five Mortality in Least Developed Countries: Longitudinal Evidence on Vaccine-Specific Moderation by Government Health Expenditure

Abstract

Background Under-five mortality remains disproportionately high in least developed countries (LDCs), despite expanded access to routine childhood immunisation. This study examined six vaccine-specific associations with under-five mortality, the moderating role of government health expenditure (GHE), and variation across the mortality distribution. Methods An ecological longitudinal panel of 41 LDCs covering 2000 to 2023 was analysed. Six vaccine-specific models were estimated separately using country fixed effects with Driscoll-Kraay standard errors. Moderation was assessed using interactions between within-country-centred vaccine coverage and GHE. Split-panel jackknife method-of-moments quantile regression evaluated effects at the 25th, 50th, and 75th conditional quantiles. Results The findings showed vaccine-specific asymmetry, with direct and GHE-moderated effects varying in magnitude, significance and direction across the six vaccines. A 1% increase in coverage of the third dose of the diphtheria-tetanus-pertussis vaccine (DTP3), measles-containing vaccine (MCV), third dose of the Haemophilus influenzae type b vaccine (Hib3), and third dose of the polio vaccine (Pol3) was associated with reductions in under-five mortality of 0.229%, 0.162%, 0.072%, and 0.243%, respectively. Coverage of the third dose of the hepatitis B vaccine (HepB3) and Bacillus Calmette-Guérin vaccine (BCG) showed no significant average association. GHE attenuated the inverse associations for DTP3, MCV, BCG, and Pol3, strengthened the HepB3 association, and did not significantly moderate Hib3. Vaccine effects were generally strongest at the upper mortality quantile. Out-of-pocket health expenditure was positively associated with mortality, whereas gross domestic product per capita and basic sanitation coverage were inversely associated. Conclusions Routine immunisation was associated with improved child survival in LDCs, but the benefits varied by vaccine, financing conditions, and mortality level. Policies should prioritise effective delivery, preventive expenditure, and high-mortality populations.